Bug 916887 - waitForEvent should return the promise that is resolved after its cleanup is complete. r=mbrubeck

--HG--
extra : rebase_source : fb9bf106058495563e3fa37802b7bb320db773bf

browser/metro/base/tests/mochitest/head.js

@@ -341,14 +341,13 @@ function waitForEvent(aSubject, aEventName, aTimeoutMs, aTarget) {
     eventDeferred.resolve(aEvent);
   }
 
-  function cleanup() {
+  function cleanup(aEventOrError) {
     // unhook listener in case of success or failure
     aSubject.removeEventListener(aEventName, listener);
+    return aEventOrError;
   }
-  eventDeferred.promise.then(cleanup, cleanup);
-
   aSubject.addEventListener(aEventName, listener, false);
-  return eventDeferred.promise;
+  return eventDeferred.promise.then(cleanup, cleanup);
 }
 
 /**
@@ -425,7 +424,7 @@ function waitForCondition(aCondition, aTimeoutMs, aIntervalMs) {
 }
 
 /**
- * same as waitForCondition but with better test output. 
+ * same as waitForCondition but with better test output.
  *
  * @param aCondition the callback that must return a truthy value
  * @param aTestMsg test condition message printed when the test succeeds or